Bonded magnetic core and process for producing it



J. G. FORD Dec0 30, 1952 BONDED MAGNETIC CORE AND PROCESS FOR PRODUCINGIT Filed Sept. 6, 1951 d. Q m F fw. T G. m S /W e T W m JT 0 J VI-coEmonEoo oczocomEH B E ow Em S xwoom zo; ul lzhzl HQIH N r \||l Il Il ITill 1 li |l\ lll \I I. Il lm \||1|| m. Il Ill Il XIIIIIII IMI.:IIIIIIIIIIHII man mm2@ o .NOE .mi

WITNESSES: f/fm Patented Dec. 30, 1952 BONDED MAGNETIC CORE AND PROCESSFOR PRODUCING IT James G. Ford, Sharon, Pa., assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania Application September 6, 1951, Serial No. 245,320

Claims.

This invention relates to bonded magnetic cores and the processes bywhich they may be produced.

In the electrical industry there are being produced a substantial numberof bonded magnetic cores. In many cases these cores comprise assembledlaminated magnetic iaterial, ordinarily preferentially oriented magneticmaterial, and the assembled laminations are bonded with suitableresinous binders and finally the bonded magnetic cores are cut intosections that may be assembled in association with electrical coils. Themanufacturing operations of cutting the bonded magnetic core oftenresult in splitting or delamination of the magnetic core and the core isthen not usable and is scrapped or reprocessed. In the handling andassembling of the cut bonded magnetic core sections, the cores may splitor delaminate, and even in service cores will occasionally separate andresult in unsatisfactory operation. In any event, a split or delaminatedmagnetic core constitutes a loss.

The bonding of laminations together to produce the best bonded cores sothat splitting or delamination does not take place to a marked extent,may be obtained if the interlaminar spaces are completely filled with agood resin binder. However, the electrical losses in resin bondedmagnetic cores have been found to be a function of the type of resinousbinder used and the amount of the resin binder present. Therefore,

if the interlaminar spaces are completely filled with a given resin, thecore losses in an alternating current field will be the highest whereasif the interlaminar spaces are only partly filled, the core losses willbe proportionately less. The minimum core losses for a given core areobtained when no resin is present. 'I'here is evidence that, because theinterlaminar spaces are so thin, when lled with a resin and cured insuch thinA sections to function as a binder for the laminations. theapplied thin sections of resin shrink and develop physical distortionsof such magnitude that they impose strains on the magnetic laminations.The magnetic laminations, particularly if preferentially oriented, areextremely sensitive to strains. Therefore, even with the best corebinders available today, core losses may be substantial.

It accordingly will be apparent that in the building of magnetic coresthe use of large amounts of resin binder so as to ll the interlaminarspaces will reduce the splitting and de- -lamination of cores duringsubsequent manufacturing and service use, but large amounts of 2 resintend to increase the core losses. This requires the manufacturing anddesign engineers to make a compromise between excessive scrapping ofcores due to splitting and delamination, and high electrical corelosses. Furthermore, even with the best practice, the normalnon-uniformities in impregnation, cutting and other steps of themanufacturing processes result in the production of a substantial amountof scrap due to A delamination and split cores. It has been a problem inthe manufacture of electrical cores to reduce such scrap to a lowproportion of the total number of cores manufactured.

'I'he object of this invention is to provide for a process for bondinglaminations of a magnetic core by, first, applying a relatively smallquantity of resin binder to effect interlaminar bonding through spots ofadhesive and then applying a wrapping of a fibrous material incombination with a heavy coating of resin to the exterior of the core toprovide for both good bonding and low core losses.

Another object of this invention is to provide for a bonded laminatedmagnetic core in which the bonding means is applied mainly to theexterior of the core and only a minor proportion of binder is applied tothe interlaminar spaces within the core.

Other objects of the invention will in part be obvious and will in partappear hereinafter. For a better understanding of the nature and objectof the invention reference should be had to the following detaileddescription and drawing, in which:

Figure l is a vertical cross section,

Fig. 2 is a view in elevation of a magnetic core,

Fig. 3 is another vertical cross section, and

Fig. 4 is a view in elevation of a completed magnetic core comprisingtwo sections.

I have discovered that bonded laminated magnetic cores having extremelylow core losses, approaching those of the unbonded magnetic cores may beheld together by bonding means applied mainly to the exterior of thecore and associated with a predetermined small amount of interlaminarresin binder so that they will withstand manufacturing and service useWith negligible delaminating and splitting. Briefly, the process forpreparing the cores comprises the following steps: first, the assembledlaminations of magnetic material are treated to apply to theinterlaminar spaces a bonding resin which when cured fills less than 18%of the interlaminar spaces; then, after curing the resinous interlaminarbinder, the exterior of the magnetic core, except for working faces, iswrapped to cover at least a. major portion thereof with anon-conductive, fibrous wrapping; and, finally, the exterior of thecore. except for the working faces, is heavily coated with a bondingresin that penetrates the iibrous wrapping. By this process I haveproduced magnetic cores whose electrical losses are very nearly the sameas the losses of the original uncut core. The scrap losses inmanufacturing and service operations are substantially completelyeliminated.

The four figures of the drawing illustrate consecutive steps followed inthe practice of the process of this invention. Referring to Fig. 1 ofthe drawing, there is illustrated the first step in which the magneticcore I comprising a plurality of assembled laminations I2. which corehas been annealed to remove strains developed during its manufacture andassembly. is immersed in a low viscosity, low resin solids impregnatingcomposition I4 maintained within a receptacle I6. The magnetic core maybe suspended from a support I8 or the magnetic core may be simplystacked within the receptacle I6 and the impregnating composition I4flowed over the cores and then withdrawn as is well known inimpregnating procedures.. The impregnatlng composition I4 preferablycomprises less than 18% of curable resin solids, the balance beingvolatile solvent. The impregnating composition may have as little ascurable resin solids. When immersed, composition I4 will penetratereadily into the interlaminar spaces between the laminations I2. Sincethe interlaminar spaces are ordinarily less than 1 mil in thickness, theimpregnating composition I4 will be retained therein by capillaryforces. The core I0 may be withdrawn from the impregnating compositionI4 and is heat treated to drive off the solvent and to cure the resin inorder to bond the laminations I2 to one another. The quantity of curedresin present between laminations will be less than 18% of theinterlaminar spaces.

I have discovered that when the quantity of applied resin is less than18%, the alternating current losses induced by any strains that may beimparted by the cured resin in the interlaminar spaces are extremelylow. When cores so prepared are split apart, it is found that the resingives spot adhesion and does not bond substantial areas of laminationsto one another. This type of spot adhesion appears to favor lowelectrical core losses.

Numerous types of resins may be employed for providing this interlaminarbonding. A thermosetting resin will ordinarily be preferred even thoughit may cause a slightly greater electrical loss than would athermoplastic resinous binder. A thermoset binder enables cores to beemployed at higher operating temperatures than is permissible withthermoplastic materials. Furthermore, resistance to hot oil or otherdielectric is superior with thermoset binders. Resinous binders givingthe lowestelectrical losses are selected for the interlaminarimpregnation.

After the impregnated core has been heat treated to cure the smallamount of applied impregnating resin in the interlaminar spaces, itsexterior is then tightly wrapped with a nonconducting fibrous material.'The fibrous material may be composed of glass ber tape, glass fibercord. strands of glass ber, cotton cord asbestos tape or tapes ofsynthetic resin fibers, for example glycol-terephthalate fibrouspolymers cover at least half of the exterior surface of the magneticcore or it may be applied to substantially completely cover the entireexterior surface of thecore, except for core faces that may be presentvin some types of cores. It is preferable that the applied fibrouswrapping be relatively open in structure in order that the next step inthe process be more eective. As shown in Fig. 2, a glass tape 22 isapplied as a iibrous wrapping to the heat treated magnetic core I0.

Referring to Fig. 3 of the drawings, the magnetic core I0 with the glasswrapping 22 is immersed in a high viscosity, surface coating resinouscomposition 24 disposed within a tank 2S. The magnetic core may besuspended by a support 28 or introduced by any suitable means into thetank 26. The resinous coating composition 24 contains a curableresinouscomposition present in such amount and of-such viscosity that itwill not substantially penetrate into the interlaminar spaces of thecore I0. However, the resinous composition will penetrate into the opentextured brous wrapping 22 and will heavily coat all of the exteriorsurfaces of the magnetic core I0 so that when the core I0 is withdrawnfrom the tank 26 a substantial thickness of the composition will bepresent on all surfaces of the magnetic core. The core I0 is then heatedor otherwise treated to cause the coating composition 24 present on allof the exterior surfaces to cure into a hard .bonding coating. Thecombination of the interlaminar binder and the brous wrapping and theapplied cured compositio'n 24 produces a bonded core that will withstandall reasonable manufacturing and service conditions.

The fully bonded magnetic core may be readily machined, ground andetched as required to produce sectioned magnetic cores having workingfaces. As illustrated in Fig. 4 of the drawing the magnetic core I0 withthe exterior coating 30 of cured resin composition and glass tape hasbeen cut into two sections 32 and 34 having machined faces 36 that willfit closely together.

Examples of the practice of the present invention are the following:

`iikxximple I A number of magnetic cores of a size to be employed in 3kv.a. 7200 volt transformers were prepared by winding 14 mil thickstrips of preferentially oriented silicon iron. The cores when woundwere in the form of loopedv cores such as is shown in the figures of thedrawing. After strain anneal, these cores were then immersed in animpregnating resinous composition comprising solvents (a mixture ofacetone and alcohol) and 15% of curable resin solids. The resin solidscomprised a mixture of (a) parts by weight of polyvinyl acetate and (b)70 parts by weight of the potentially reactive, partial reaction productof aniline, Phenol and formaldehyde reacted in the proportions 160 partsof aniline, 533 parts of phenol, 200 parts of paraformaldehyde and partsby weight of 40% formaldehyde. After the cores were impregnated in thecomposition, they were then baked for 2 hours at 200 C. to drive oif thesolvent and to cure the resin in the interlaminar spaces. The bakedcores were then wrapped with an open weave glass tape 1 inch in width inwhich the turns of the wrapping were spaced approximately 1A" apart atthe outside of thelooped core. The wrapped cores were then immersed andnylon. The applied iibrous wrapping should 75 in a high viscosityresinous epoxide having '10% resin solids. The viscosity of the resinousepoxide composition was over 1000 centipoises. It formed a heavy coatingand impregnated into the open weave glass cloth without penetratingappreciably into the interlaminar spaces. The cores were then cured byheating at a temperature of 100 C. for 1 hour. However, this epoxideresin could be modified by the addition of -a higher proportion of anamine catalyst so that it would cure at room temperature in 6 hours.Thereafter the completed core was cut into two sections as required formaking of a transformer therewith, and was tested for electrical losses.The average true watts per pound of magnetic core, after winding butbefore any impregnation, was 0.689. After the process of impregnation,bonding and cutting was completed, the average true Watts per poundvalue was 0.701. This change in loss is lessthan 2%. Of the entireseries of cores prepared, none delaminated or split and all exhibitedexcellent physical and electrical properties.

Example II Another series of wound magnetic cores of the shape shown inthe figures of the drawing was prepared by impregnating the interlaminarspaces with a resinous composition comprising of curable resin solidsand 85% of solvent.,

The resin solids comprised 67 parts by Weight of a potentially reactiveresorcinol formaldehyde resin disclosed in Example II of Patent2,542,048, and 200 parts of polyvinyl acetate of medium molecularweight. After curing the applied resin composition, the magnetic coreswere all wrapped with open weave glass tape in the manner set forth inthe preceding example. The cores were then dipped in a similar epoxideresin to that used in Example I. After the curing of the applied epoxideresin, the wound cores were each split into two U-shaped sections andthe faces were ground and etched. The magnetic cores were then testedfor electrical losses. In this series of cores, the average true wattsper poundA was 0.687 before the cores were impregnated, while after theentire bonding and cutting process of this invention, the average trueWatts per pound was 0.665. This is a reduction in losses ofapproximately 3%.

The resinous epoxide composition employed in the examples was preparedas follows.

Four moles of 4,4dihydroxy diphenyl-2,2 propane and five moles ofepichlorhydrin were added to an aqueous caustic soda solution containing6.43 moles of sodium hydroxide. The reaction mixture was heated slowlyfrom an initial temperature of 40 C. to 100 C. in 80 minutes. Reactionwas continued for one hour at a temperature of from 100 C. to 104 C. Thereaction mixture was then permitted to stand until it separated into twolayers. An upper aqueous layer was drawn oif and discarded. The lowerlayer containing the resinous reaction product was first washed withwater several times and the water withdrawn to remove excess caustic.Dilute acetic acid was then stirred in to neutralize unreacted caustic.Further washing in water was carried out until the wash water wasneutral to litmus. The product was freed from water by decantation andthen heated to 130 C'. to

eliminate all traces of water. The resinous product so formed had asoftening point of 100 C. using Durrans mercury method.

The resinous epoxide so prepared was dissolved in a solvent, such asacetone. methyl ethyl ketone or ethyl acetate, or a mixture of solvents,to

produce the solution of surface coating resinous composition, The resinmay be catalyzed by adding a small amount (0.1% to 4%) of an aliphaticamine, such as diethyi amine, or an alkali such as potassium hydroxideor sodium phenoxide.

Numerous other resins may be employed both for the impregnatingcomposition and for the surface coating resinous composition of highviscosity as disclosed herein. Thus the magnetic core may be initiallyimpregnated with a low viscosity solution prepared in accordance with myPatent 2,372,074. 'I'he surface coating resinous composition may includehigh viscosity, heat hardening resinous compositions such asmelamine-formaldehyde resins, aniline-phenol formaldehyde resinscombined with thermoplastic additives, and resorcinol-formaldehyde resinwith vinyl polymers added thereto as disclosed in Nagel Patent2,542,048. Again, both the impregnating composition and the surfacecoating composition may be a polysiloxane resin. 'I'hus the impregnatingcomposition may be a heat hardening low viscosity, low solids contentphenyl methyl siloxane and the surface coating composition may be a heathardening, high viscosity phenyl methyl siloxane or other polysiloxaneresin.

While the drawing has been illustrated with reference to a looped woundcore comprising a plurality of turns of magnetic material, the inventionmay be applied to other magnetic core structures either ofparallelepiped form or of other shape. The magnetic cores may comprisenot only sheets of magnetic material but ribbon, wire or other shapedmagnetic material that may be assembled into a magnetic core.

Regardless of the shape or size of the magnetic core the processesdisclosed herein will enable the strongest bonded magnetic core withlowest electrical losses of any produced heretofore.

Since certain changes may be made in the above invention and differentembodiments of the invention may be made without departing from thescope thereof, it is intended that all matter contained in the abovedisclosure shall be interpreted as illustrative and not in a limitingsense.

I claim as my invention:

1. In the process of producing a bonded laminated magnetic core, thesteps comprising assembling laminations of magnetic material to form acore, impregnating the spaces between the laminations of the assembledcore with a low viscosity penetrating solution of a resin, the solutionhaving a curable resin solids content of less than 18% by weight and thebalance of the solution being solvent, heat-treating the impregnatedcore to drive off the solvent and to cure the applied resin to bond thelaminations to one another, the amount of applied cured resin beinginsufficient to bond the laminations into a core capable of withstandingsubsequent manufacturing and service use, wrapping a substantial portionof the exterior of the heat-treated core with nonconducting fibrousmaterial, applying a viscous and substantially non-penetrating curableresinous composition to the wrapped laminated core to coat the exteriorsurface of the core without penetrating the interlaminar spaces and tocoat the fibrous Wrapping material, and curing the last-mentionedapplied resinous composition, the laminations of the resulting magneticcore being so well bonded that the core will acea'aso withstandmanufacturing and service use and the quantity of resin betweenlaminations being so low as to cause low magnetic losses in service.

2. In the process of vproducing a wound mag-` netic core, the stepscomprising winding sheet magnetic material into a looped magnetic corehaving a plurality of turns, impregnating the interlaminar spacesbetween turns of the wound core with a low-viscosity penetratingsolution of a resin, the solution having a curable resin solids beinginsuilicient to bond the turns into a looped core capable of being cutand withstanding subsequent manufacturing and service use, wrapping asubstantial portion of the exterior surface of the looped core with anon-conducting fibrous material, applying a viscous and substantiallynon-penetrating resinous curable composition to the wrapped looped coreto coat all the exterior surface of the core without penetrating theinerlaminar spaces between turns and to coat the fibrous wrappingmaterial, curing the last-mentioned applied resinous composition,cutting the resulting wound core to produce two U-shaped sections, thecombined resins and wrapping material providing a wound core that may becut and further processed to produce apparatus withoutY delaminating andcharacterized by 10W magnetic losses.

3. A bonded, integral magnetic core comprising a plurality of assembledlaminations of magnetic material, resinous bonding material appliedbetween laminations, the resinous bonding material filling less than 18%of the interlaminar spaces,

a non-conducting fibrous material wrapped about a major proportion ofthe exterior surface of the magnetic core, a relatively heavy coating ofresin applied to the exterior surface, the brous wrapping material andthe heavy coating of resin applied to the exterior of the core providingfor bonding the laminations into'an integral core.

4. A bonded; integral magnetic core section having a plurality ofassembled laminations with at least one working face, resinous bondingmaterial applied between laminations, the resinous bonding materialfilling less than 18% of the interlaminar spaces. a non-conductingfibrous material wrapped about a majorproportion of the exterior surfaceof the core section except for the working faces, a lrelatively heavycoating of resin applied to all of the exterior surface of the coresection, except the working faces, and to the fibrous wrapping material,the fibrous wrapping material and the heavy coating of resin applied tothe exterior of the core section providing for bonding the laminationsinto the integral section capable of withstanding manufacturing andservice use without delamination.

5. The core section of claim 4 wherein the fibrous wrapping materialconsists of glass bers.

.mams G. FORD.

REFERENCES (CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,586,889 Elmen June 1, 19262,479,417 Schulman et al. Aug. 16, 1949 2,579,560 Ford v Dec. 25, 1951

3. A BONDED, INTEGRAL MAGNETIC CORE COMPRISING A PLURALITY OF ASSEMBLEDLAMINATIONS OF MAGNETIC MATERIAL, RESINOUS BONDING MATERIAL APPLIEDBETWEEN LAMINATIONS, THE RESINOUS BONDING MATERIAL FILLING LESS THAN 18%OF THE INTERLAMINAR SPACES, A NON-CONDUCTING FIBROUS MATERIAL WRAPPEDABOUT A MAJOR PROPORTION OF THE EXTERIOR SURFACE OF THE MAGNETIC CORE, ARELATIVELY HEAVY COATING OF RESIN APPLIED TO THE EXTERIOR SURFACE, THEFIBROUS WRAPPING MATERIAL AND THE HEAVY COATING OF RESIN APPLIED TO THEEXTERIOR OF THE CORE PROVIDING FOR BONDING THE LAMINATIONS INTO ANINTEGRAL CORE.